1. Technical Field
The present invention relates to an image recording apparatus and more particularly to an image recording apparatus including a recording head such as a droplet ejecting head each provided with plural nozzles for ejecting liquid droplets for recording an image.
2. Related Art
Well known conventionally are ink jet recording apparatuses (so-called ink jet printers) having an ink jet recording head which cause changes in the volume (expansion/contraction) of pressure generating chambers filled with ink, using an actuator constituted of piezoelectric device and the like, so as to eject ink droplets from the front end of a nozzle communicating with the pressure generating chamber.
Recently, there is an increasing tendency to make ink jet recording apparatuses smaller and increase printing speeds. Thus, ink jet recording heads are being used which are capable of forming an image over a wide area in as short a time as possible by extending the length thereof, increasing the number of nozzles per ink jet recording head and arranging the nozzles in line.
Such an ink jet recording apparatuses adopt roll conveying methods, in which plural conveying rolls are disposed along a conveying direction (conveying path) and driven to transport a recording medium, and belt conveying methods, in which a conveying belt is wound around drive rolls at both ends and moved by driving the drive rolls in order to transport a recording medium or the like.
In the roll conveying methods, the conveying speed of the recording medium changes due to changes in rotation speed of each conveying roll. In the belt conveying method, the conveying speed of the recording medium changes because of unevenness of thickness of the conveying belt or the accuracy of components such as the circularity of the drive roll or the like. As a consequence, a recording medium conveyed at a changed conveying speed passes the recording head at the changed speed so that an image formed with droplets ejected from the recording head at a predetermined timing is deformed. For example, part or whole of the image is expanded or contracted in the conveying direction or unevenness in density occurs. When a color image is produced by overlapping plural colors, due to misalignment between respective images or unevenness of density within a single color image, smudged colors occur leading to reduction of the quality of color images.
To solve this problem, is known a technology for adjusting printing timing, by detecting a belt surface velocity by providing a sensor in the vicinity of each print head for recording a color image on the recording medium or by detecting the velocity of the belt surface with the interval between respective print heads set to a distance equal to the peripheral length of the drive roll.
There has been also a technology which aims at improving recording velocity by using a recording head in which nozzles are arranged two-dimensionally. With two-dimensional arrangement of the nozzles, if the actual conveying velocity of the recording medium does not coincide with the prescribed conveying velocity determined with respect to a standard image recording timing specified as the interval between recorded dots by adjacent nozzles in the conveying direction, that is, if the distance traveled at the conveying velocity between the image record timings of the adjoining nozzles does not coincide with the interval of recorded dots by the distance between the nozzles (nozzle pitch), disjointing of the lines in a main scanning direction is generated (unevenness in print density). Consequently, bad quality characters and line drawing occurs.